Silent Information Regulator 2 (Sir2) proteins, or Sirtuins, are a family of evolutionally conserved enzymes with nicotinamide adenine dinucleotide (NAD)-dependent protein deacetylase activity (A. A. Sauve, et al., Annu. Rev. Biochem. 75:435 (2006); S.-i. Imai, et al., Trends in Pharmacological Sciences 31:212 (2010); M. C. Haigis, et al., Annual Review of Pathology: Mechanisms of Disease 5:253 (2010)). Since the initial discovery of sirtuin deacetylase activity (S.-i. Imai, et al., Nature 403:795 (2000); K. G. Tanner, et al., Proc. Natl. Acad. Sci. U.S.A. 97:14178 (2000)), many important biological functions of sirtuins have been revealed, and the NAD-dependent deacetylation mechanism is well understood (S.-i. Imai, et al., Trends in Pharmacological Sciences 31:212 (2010); M. C. Haigis, et al., Annual Review of Pathology: Mechanisms of Disease 5:253 (2010)). Based on sequence similarity, sirtuins can be grouped into different classes (R. A. Frye, Biochem. Biophys. Res. Commun. 273:793 (2000)). Mammals have seven sirtuins, Sirts1-7. Sirts1-3 belong to Class I, Sirt4 belongs to Class II, Sirt5 belongs to Class III, and Sirt6 and Sirt7 belong to Class IV (R. A. Frye, Biochem. Biophys. Res. Commun. 273:793 (2000)).
A major obstacle in the study of sirtuin activity is the fact that four of the seven human sirtuins (Sirts4-7) have either very weak or no deacetylase activity (E. Michishita, et al., Mol. Biol. Cell 16: 4623 (2005); M. C. Haigis et al., Cell 126:941 (2006); A. Schuetz et al., Structure 15:377 (2007); G. Liszt, et al., J. Biol. Chem. 280:21313 (2005); E. Michishita et al., Nature 452:492 (2008)). This poses many difficulties for the study of Sirtuins and development of small molecules that can modulate their activity. For example, it is difficult to develop inhibitors or activators that target Sirts4-7 because no robust activity assay is available. Also, it is hard to tell whether the inhibitors/activators that target Sirt1, Sirt2, and Sirt3 can also target Sirts4-7.